Add support for polymorphic hasMany associations.

Saving async polymorphic associations is not currently supported.

Also fixes the payload required by the ActiveModelAdapter to match the
Rails ActiveModel Serializers.

```js
  MyUser = DS.Model.extend({
    name: DS.attr('string'),
    messages: DS.hasMany('myMessage', {
      polymorphic: true,
      inverse: 'user'
    })
  });

  MyMessage = DS.Model.extend({
    name: DS.attr('string')
    comments: DS.hasMany('myComment', {
      inverse: 'message'
    }),
    user: DS.belongsTo('myUser')
  });

  MyPost = MyMessage.extend({
  });

  MyComment = MyMessage.extend({
    message: DS.belongsTo('myMessage', {
      polymorphic: true,
      inverse: 'comments'
    })
  });
```

The expected payload for a polymorphic `belongsTo` association is then:

```js
  my_comment: [{
    id: 2,
    name: "Rails is omakase",
    message: {
      type: 'my_post',
      id: 1
    }
  }],

  my_posts: [{
    id:1,
    name: "OMG"
  }]
```

The expected payload for a polymorphic `hasMany` association is then:

```js
  my_user: [{
    id: 1,
    name: "Cyril Fluck",
    messages: [{
        type: 'my_post',
        id: 1
      }, {
        type: 'my_comment',
        id: 2
    }]
  }],

  my_posts: [{
    id:1,
    name: "OMG"
  }],

  my_comments: [{
    id: 2,
    name: "Rails is omakase"
  }]
```